Prussian Blue

What it is

Prussian blue is iron(III) hexacyanoferrate(II): a cubic framework in which iron atoms in two oxidation states are bridged by cyanide groups. Its intense colour comes from intervalence charge transfer — light in the orange-red drives an electron from one kind of iron to the other — which is why so little pigment gives such a deep, cool blue. Because the colour depends on that oxidation-state pairing, anything that reduces or disrupts the framework takes the colour away.

How it was obtained

It was discovered by accident in Berlin around 1704–06, when a colour maker's red lake preparation, made with contaminated alkali, turned blue. Early manufacture relied on alkali prepared by heating animal matter such as dried blood with potash, then precipitating with iron salts; later industrial routes used purer iron and ferrocyanide compounds. As the first modern synthetic pigment and far cheaper than ultramarine, it spread rapidly across Europe and later to Japan, where it transformed woodblock printing.

How it ages

Pure Prussian blue is reasonably lightfast, but it fades in some circumstances. National Gallery research found fading mainly where it was mixed with a lot of white, and early or potassium-rich grades were less permanent; faded 18th-century skies look grey in cross-section. It can also partly recover in the dark as the iron is re-oxidised. Alkalis destroy it, leaving brown iron hydroxide, so it was unsuitable for fresco and can suffer beside alkaline materials.

How it is identified today

The cyanide bridge gives a sharp band in infrared and Raman spectra in a region where almost no other pigment absorbs, so vibrational spectroscopy identifies it even in small amounts. XRF shows only iron, which is not diagnostic alone. Microscopy shows very fine, agglomerated particles that look almost like a dye, and SERS has detected it mixed with indigo in paintings.

What replaced it

Prussian blue is still made and sold, as Prussian or Milori blue, and remains common in artists' paint and some printing inks. Phthalocyanine blue, introduced in the 1930s, has largely taken its place for a strong, cool blue: it is similar in hue and strength but more lightfast in tints and stable to alkali, so it does not suffer the same fading when mixed with white.

What the evidence says

• FactStrong evidence

  Prussian blue in paintings has faded mainly where mixed with a lot of white; early and potassium-rich grades were less permanent, and faded areas look grey in cross-section.

  Source: Fading and Colour Change of Prussian Blue: Occurrences and Early Reports

• FactModerate evidence

  Prussian blue is decomposed by dilute alkalis to brown iron hydroxide, and its lightfastness falls when it is mixed with other pigments.

  Source: Prussian blue — ColourLex pigment page

• FactStrong evidence

  Faded blue in a colonial American portrait proved to be indigo mixed with inexpensive Prussian blue, identified by SERS.

  Caveat: A single painting.

  Source: Uncovering Art's Vanishing Hues with Surface-Enhanced Raman Scattering: Drawing Inspiration from the Past for the Future